The invention relates to a method and apparatus for balancing of workpieces by removal of surface portions thereof.
Many types of workpieces, particularly rotors, must be balanced after manufacture and before they can be used. This is particularly true of rotors such as electric motors which must rotate at high speeds and therefore must be well balanced in order to avoid excessive noise, vibration and other undesirable effects.
Typically, these kinds of rotors are balanced by conventional machines which determine the angle and amount of imbalance and calculate where material should be removed in order to balance the rotor. The rotor is then automatically or manually positioned with respect to a mill, drill, grinder or the like which is then advanced to contact the desired position or positions or vice versa and to remove the desired amount of material.
Typically, the amount of material removed is varied by varying the depth to which drilling, grinding or milling takes place. However, in order to accurately determine the depth, it is necessary to accurately determine when the drill or mill contacts the surface of the rotor. One way to do this is to isolate the tool and rotor from ground and apply a D.C. voltage therebetween. The electrical connection which takes place when the metallic tool touches the metallic rotor can be easily detected to provide an indication of contact. However, the difficulty with this arrangement is that the chips which are removed from workpieces frequently will bridge between the tool and a workpiece providing an inaccurate indication of contact. Since the depth of drilling must be very accurate to effectively balance the workpiece, this small error leads to workpieces which are entirely unsatisfactory. While the use of an AC current and an adjacent induction coil to determine when current flows between the tool and the rotor avoids problems arising from trying to isolate the tool and workpiece holder, the same problem of bridging by metallic chips still exists.
Another alternative which has been utilized is to mount the tool or workpiece holder in a spring holder so that it presses backward to close a mechanical switch at the time of contact. This arrangement too has a number of disadvantages. The spring-loaded holding arrangement is mechanically complex and may require modification of conventional drills, mills, grinders and other tools. Moreover, the mechanical switch can fail and the spring is likewise subject to wear.
The present invention relates to an improved apparatus of this type in which a force transducer is coupled to the workpiece or tool holder, at a position so that it produces an electrical signal in response to the force resulting from contact between the tool and the workpiece. A force transducer is a relatively inexpensive and reliable device. Moreover, by coupling that device to the workpiece holder, no structural modification need be made in the tool which removes material from the workpiece.
In a first embodiment of the invention, the force transducer, for example, a piezoelectric transducer, is mounted between a base member and a clamping member constituting the workpiece holder. The rotor to be balanced is clamped between a pair of jaws and the members are connected together by a threaded screw or the like which is preloaded by a spring. The force transducer is mounted between the members adjacent the screw so that when the drill contacts the surface of the rotor at the position where material is to be removed, the resultant change in force applied to the force transducer produces a change in voltage. This voltage change is then applied through an amplifier which operates as a kind of level detector to produce an output signal only when a threshold voltage has been exceeded. The output of this amplifier in turn sets zero in a conventional feed motor controller which responds to a signal from a suitable conventional balancing circuit to produce an output to the running feeding motor to further advance a drill for a predetermined distance which will remove the desired amount of material and effect balancing of the rotor. The drill can approach the rotor in fast feed until the rotor is touched and the switch to slow speed which saves time in an automated process.
According to a second embodiment of the invention, the force transducer is similarly mounted in a holder which includes a mill spindle and cutter against which the workpiece holder and the workpiece are advanced. Each time a tooth of the mill contacts the surface of the rotor, a pulse will be produced by the force transducer which is applied to filter and amplifier circuit tuned to the frequency of teeth of the mill cutter and the speed at rotation thereof. Thus, the amplifier passes the pulse train only when its frequency indicates that the signal comes from the operation of the mill. A level detector then determines whether the pulses are of sufficient amplitude. A stepper motor is then similarly controlled by a conventional control circuit to advance the rotor and remove the desired material.